1. Field of the Invention
This invention relates to a recording method and apparatus for effecting recording of images on a recording medium by the use of heat.
The term "recording apparatus" covers apparatuses having the recording function, such as an electronic typewriter, a printer, a facsimile apparatus and a copying apparatus.
2. Related Background Art
Description will hereinafter be made with a heat transfer printer taken as an example of the recording apparatus.
Generally, in the thermal head of a heat transfer printer or the like, the efficiency of energy transmitted to an ink sheet is very low for the energy applied to the thermal head, and the value thereof is said to be about 50% or less. This loss of heat is due to the transmission or radiation of heat to the base material of the thermal head and into the air. Like this, the energy applied to the thermal head is not all used for the heating of the ink sheet, but the amount of heat generated by the thermal head is very greatly affected by the environment in which the apparatus is placed and the data recorded.
So, as a method of controlling the amount of heat generated by a heat generating member in such a thermal head, there has been practiced the history correction in which the energy applied to the thermal head is adjusted on the basis of the recording information for several lines in the past, or the accumulated heat correction in which the general temperature rise of the glaze layer of the thermal head is foreseen on the basis of the recording data and the energy applied to the thermal head is adjusted.
Of these, a popular method used in the latter accumulated heat correction is such that in the case, for example, of a serial type heat transfer printer, how many black dots have existed between the head of the currently printed line to the line to be printed next is counted and the applied energy is controlled stepwise on the basis of the counted value. Accordingly, if spaces are included in that line, the applied energy is increased at a predetermined ratio in conformity with the amount of the spaces. The purpose of such accumulated heat correction is to keep the recording density in that line uniform and further keep the recording density from the beginning to the end of each page uniform.
However, the conventional accumulated heat correction method has suffered from the problems shown below.
[1] When the recording (printing) speed is low
For example, if an underlined character row is printed and thereafter only the underline is printed, the print of the underline portion becomes thin. This is because the underline is continuous and therefore the portions corresponding to spaces become absent and due to the accumulated heat correction process, the energy applied to the thermal head continues to fall stepwise.
[2] When the recording (printing) speed is high
(a) The period of energy application to the thermal head becomes faster and therefore, as compared with the case of printing of ordinary documents, if recording of figures or graphics is effected, a sudden change in the recording dot density cannot be accommodated and thus, the recording density becomes too high. PA1 (b) Also, for the same reason as in the case of item (a) above, owing to the difference in character pitch or character font, or further if bold characters, double-width characters or quadruple square characters are printed, the recording density of those characters becomes high. PA1 (c) The control of the recording density effected on the basis of the number of black dots is usually executed under the control of a CPU or the like and therefore, if the recording speed becomes higher, the burden of the CPU increases. Therefore, in the conventional accumulated heat correction, the recording speed cannot be increased beyond a certain speed, and so there has been an upper limit on the speed.